With my normal-range prime lenses, I tend to shoot with both eyes open — what I see in the viewfinder has a high correspondence to what I see with my other eye. With my 70mm (105mm in 35mm-equivalent terms), it's a little trickier, but not so much that I really ever have to think about it.

With the Pentax DA★ 200mm f/2.8, though, it's a different story, for two reasons. First, at 300mm-equivalent, the field of view is so radically smaller than my unaided vision that it's just confusing to operate with two eyes. And second, the depth of field is so small that if I move away from my target, there's good odds I'm just looking at blur.

If this were a zoom lens, I might consider tracking the scene at a wider focal length and then zooming in. But that's not an option.

Clearly, people use these lenses for sports and wildlife photography all the time. What's the trick? I was at a not-so-fast-moving kung fu demonstration, and kept losing people. I can't imagine catching a bird in flight; do people just point at a flock and hope something is in the field of view? Is this just a matter of really learning to know the lens, or is there something I can do?

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Based on the last two answers I may be missing the point of the question here, but instead of worrying about aperture, your question more directly relates to how do I track action.

Tracking abilities I would split into 3 categories.

The Camera

The Lens

You

Part 1: The Camera

Some cameras just wont do tracking particularly well. You really need one that can do continuous auto-focus (not all do). This is the AI Servo mode if you're in Canon land, or on Nikon there is a C/S/M setting that does this (you want C for Continuous). I'm afraid I can't comment on Pentax, but refer to your manual. So set your camera to continuously focus.

The next thing to check is to ensure it is using all available AF points. Again, this varies by camera, but you may have a diamond of 7 or 9, or be lucky enough to have more - 19, 51, 61.....depending on make and model of camera. Just make sure all of them are enabled.

Also, think about the metering mode you choose. for things against the ground, or landscapes, you may want evaluative or full metering, but for a bird or aircraft in the sky, choose centre-weighted or spot metering.

Finally, choose your shooting mode appropriately. For action shots you will want shutter priority mode. I think Pentax use the mnemonic "Tv", as do Canon. Nikon and others just use "S". Then set a shutter speed appropriate to your action. Fast moving = 1/1000th or more. Slow subjects = 1/250th maybe? The benefit of your fast lens will be felt as you can keep the ISO low whilst still getting correctly exposed shots.

Part 2: The Lens

I'm not sure this pertains to you as its about image stabilisation, and I think the Pentax uses in body stabilisation. But in the Canon/Nikon world long lenses with IS/VR modes also have a stabiliser "mode". If you have this, ensure the mode is set correctly for your shooting type. Mode 1 is generally for overall use, handholding, normal shooting. Mode 2 usually only tries to correct vertically, so is useful for panning. Perhaps in the body of the Pentax you can select the mode too??

Also, the AF of the lens must be sufficiently fast to keep up with the constant refocussing by the camera body. But the lens you suggest has an SDM motor which seems to be well liked and fast.

Part 3: You

Practice your panning. Practice following targets across the ground and sky. It doesn't have to be in the dead centre of the frame but there's nothing worse than a raft of photos of your favourite car, plane, bird, etc with it's back end cut off! You may find a change of stance helps, or to hold the lens differently, or use a Monopod. Also, I would not tend to keep both eyes open as when you are tracking action, it only serves to distract you. Concentrate on whats in the viewfinder as thats what the camera sees.

Thanks Mike. I've got a couple of followup comments. Part 1) Pentax does have an AF-C continuous mode, and while it's very poor at tracking objects moving directly towards or away from the camera (this is what people mean when they complain about Pentax AF, btw), it's just fine for lateral movement. But my problem is that if I move off the subject and the AF point grabs the background, everything becomes a blur and I lose what's going on. Part 2) Pentax claims that their system automatically does the right thing while panning. Part 3) What if the subject is moving unpredictably?
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mattdmMar 30 '12 at 15:48

@mattdm - Honestly, if the subject is moving erratically, you're pretty much SOL (or trusting entirely to the "L" part of the equation). "Erratic", though, is relative: if you're shooting wildlife, behavioural experience with the fauna will educate your guesses; knowing, to the point of grokking, the sport you're shooting will have the same sort of effect in sports photography. You won't always be right, but if you know how to read the play, so to speak, your percentage of successes goes way up. Work on your "quick draw" technique, and get to know what you're shooting as well as you can.
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user2719Mar 30 '12 at 17:40

Thanks @forsvarir. I hadn't seen that particular post, but I do have my camera configured that way — it's a good suggestion.
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mattdmMar 30 '12 at 18:31

@mattdm The issue with your AF-C mode tracking objects poorly moving directly towards or away will be something over which you have no control. Learn to work within the limits of your equipment and not expect more than it can deliver. If you absolutely need that ability perhaps it is time to look at upgrading the camera body to one that can handle what you need from it? For AF instantly grabbing a new subject, again don't know about Pentax, but my Canon 7D has a setting for the speed of the AF locking, which for tracking stuff I set to SLOW, so it doesn't jerk back and forth.
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MikeMar 30 '12 at 22:19

Shooting birds in flight with the Pentax DA★ 300mm f/4, I think the answer is twofold:

Prefocus on something, either by finding a target at a similar distance, or by the distance scale. For most larger birds at a good distance, infinity is good enough to see a bird-blob in the viewfinder.

Find the target with one eye open. Yeah, I know you've tried. Try again. I just keep my left eye open during target acquisition, and then I close it. What you're trying to do is get the bird in your naked eye to line up with the center of the viewfinder (i.e. the center focus point). As the "naked eye bird" approaches the center of the field, the "viewfinder bird" will zoom into the frame at the last moment. Then you can close your naked eye and concentrate on tracking the bird in the viewfinder and focusing.

This is the sort of thing where practice really helps.

Note: Don't engage autofocus (shutter half-press or AF button, however you have it set up) until you're sure the target is under an AF point. Otherwise, it's likely that the lens will seek to the near limit, and you have no hope of catching anything until you start at the beginning again.

Just practice a bit more, it'll come to you. One thing that I've noticed is that watching the subject isn't always enough of an indicator to when something will enter the camera's view -- if possible, use the background and other items in the scene, too. It's tough to tell at such a shallow DOF, but you may be able to make out that the blur in the background you see through the viewfinder is that big tree you see with your other eye. As the subject nears the tree, you know to be ready.

I have trouble with this too, but I find opening the lens up as wide as it will go, or until I can find the subject I am trying to capture, focusing on it, then zooming in, works for me. If you are near somewhere where seagulls hang out, they make perfect models for practicing, especially if you have an assistant with seagull treats!